1. Field of the Invention
The present invention relates to a novel aromatic polyamide having an improved heat resistance, solubility, and processability and, more specifically, it relates to a novel soluble heat-resistant aromatic polyamide in which discoloration by oxidation is prevented by a terminal end treatment of a soluble heat-resistant polyamide.
2. Description of the Related Art
Aromatic polyamides, which have molecular structures having rigid aromatic rings and amide groups with strong intermolecular cohesive forces linked together, have higher softening points and heat decomposition points. Also, they have excellent heat resistance and chemical and physical properties, such as chemical resistance, mechanical characteristics, and electrical characteristics. Therefore, aromatic polyamides are very useful materials, particularly for fibers, films, paints, adhesives, and molded articles, for which heat-resistance is needed.
Usually, aramides such as Kevlar have the characteristics described above, but have a processability drawback in that they are not soluble in most solvents. Accordingly, a method is needed for overcoming this processability problem, by making the aramide soluble.
Korshack et al report on soluble polyamides in Journal of Macromolecular Science (J. Macromol. Sci., Rev. Macromol Chem., C11, 45, 1974), but do not mention the prevention of oxidation and the prevention of discoloration caused thereby.
Generally, aromatic polyamides have a high rigidity and a high symmetrical structure, and also have excellent mechanical characteristics and higher melting points, which are approximate to decomposition points, and therefore, they can be melt-molded only with difficulty. Further, they have a poor solubility, and thus have a drawback in that they cannot be easily applied as industrial materials.
Poly(p-phenyleneterephthalamide), which is a typical aromatic polyamide, can be dissolved in concentrated sulfuric acid or hexamethylphosphorylamide or N-methyl pyrrolidone having lithium chloride, calcium chloride, etc., dissolved therein. However, this polyamide has a very poor solubility and, therefore, is disadvantageous in that it can be used only with difficulty.
Also, the amino group directly bonded to an aromatic ring is very susceptible to oxidation. For example, it is well known that aniline, which is colorless and transparent when distilled, is rapidly oxidized and thus discolored. In the present invention, the 9,9-bis(4-aminophenyl)fluorene to be used as the starting material has two aniline groups, that is, amino groups directly bonded to the aromatic ring. For this reason, discoloration occurs during the polymerization reaction, and discoloration will also occur after polymer formation due to oxidation of the terminal amino groups by light, heat or air. Thus, the discoloration of polyamides not subjected to terminal end treatment will be gradually increased.